Hereinafter, a “Q” prefix in a word of phrase is indicative of a reference of that word or phrase in a quantum computing context unless expressly distinguished where used.
Molecules and subatomic particles follow the laws of quantum mechanics, a branch of physics that explores how the physical world works at the most fundamental levels. At this level, particles behave in strange ways, taking on more than one state at the same time, and interacting with other particles that are very far away. Quantum computing harnesses these quantum phenomena to process information.
The computers we use today are known as classical computers (also referred to herein as “conventional” computers or conventional nodes, or “CN”). A conventional computer uses a conventional processor fabricated using semiconductor materials and technology, a semiconductor memory, and a magnetic or solid-state storage device, in what is known as a Von Neumann architecture. Particularly, the processors in conventional computers are binary processors, i.e., operating on binary data represented in 1 and 0.
A quantum processor (q-processor) uses the odd nature of entangled qubit devices (compactly referred to herein as “qubit,” plural “qubits”) to perform computational tasks. In the particular realms where quantum mechanics operates, particles of matter can exist in multiple states—such as an “on” state, an “off” state, and both “on” and “off” states simultaneously. Where binary computing using semiconductor processors is limited to using just the on and off states (equivalent to 1 and 0 in binary code), a quantum processor harnesses these quantum states of matter to output signals that are usable in data computing.
Conventional computers encode information in bits. Each bit can take the value of 1 or 0. These 1s and 0s act as on/off switches that ultimately drive computer functions. Quantum computers, on the other hand, are based on qubits, which operate according to two key principles of quantum physics: superposition and entanglement.
Superposition means that each qubit can represent both a 1 and a 0 at the same time. Entanglement means that qubits in a superposition can be correlated with each other in a non-classical way; that is, the state of one (whether it is a 1 or a 0 or both) can depend on the state of another, and that there is more information that can be ascertained about the two qubits when they are entangled than when they are treated individually.
Using these two principles, qubits operate as more sophisticated processors of information, enabling quantum computers to function in ways that allow them to solve difficult problems that are intractable using conventional computers. IBM has successfully constructed and demonstrated the operability of a quantum processor using superconducting qubits (IBM is a registered trademark of International Business Machines corporation in the United States and in other countries.)
A superconducting qubit includes a Josephson junction. A Josephson junction is formed by separating two thin-film superconducting metal layers by a non-superconducting material. When the metal in the superconducting layers is caused to become superconducting—e.g. by reducing the temperature of the metal to a specified cryogenic temperature—pairs of electrons can tunnel from one superconducting layer through the non-superconducting layer to the other superconducting layer. In a qubit, the Josephson junction—which functions as a dispersive nonlinear inductor—is electrically coupled in parallel with one or more capacitive devices forming a nonlinear microwave oscillator. The oscillator has a resonance/transition frequency determined by the value of the inductance and the capacitance in the qubit circuit. Any reference to the term “qubit” is a reference to a superconducting qubit circuitry that employs a Josephson junction, unless expressly distinguished where used.
The information processed by qubits is carried or transmitted in the form of microwave signals/photons in the range of microwave frequencies. The microwave signals are captured, processed, and analyzed to decipher the quantum information encoded therein. A readout circuit is a circuit coupled with the qubit to capture, read, and measure the quantum state of the qubit. An output of the readout circuit is information usable by a q-processor to perform computations.
A superconducting qubit has two quantum states—|0> and |1>. These two states may be two energy states of atoms, for example, the ground (|g>) and first excited state (|e>) of a superconducting artificial atom (superconducting qubit). Other examples include spin-up and spin-down of the nuclear or electronic spins, two positions of a crystalline defect, and two states of a quantum dot. Since the system is of a quantum nature, any combination of the two states are allowed and valid.
For quantum computing using qubits to be reliable, quantum circuits, e.g., the qubits themselves, the readout circuitry associated with the qubits, and other parts of the quantum processor, must not alter the energy states of the qubit, such as by injecting or dissipating energy, in any significant manner or influence the relative phase between the |0> and |1> states of the qubit. This operational constraint on any circuit that operates with quantum information necessitates special considerations in fabricating semiconductor and superconducting structures that are used in such circuits.
A microwave isolator is a device that allows microwave light waves to pass through it without significant amplitude attenuation (propagate) in one direction, and prohibits or significantly attenuates the microwave light waves when attempting to pass through it in the opposite direction. A reference herein to an “isolator” is a reference to a microwave isolator. In other words, the isolator operates as a directional gate for microwave light, meaning the response of the device is dependent on the direction in which the microwave light is propagating through the device. Isolators are used in quantum computing for guiding microwave signals into and out of the quantum processor in a specified flow direction.
A multi-path interferometric Josephson isolator based on nondegenerate three-wave-mixing Josephson devices is hereinafter compactly and interchangeably referred to as Multi-Path Interferometric Josephson ISolator (MPIJIS). An MPIJIS device can be implemented as a microwave isolator in a superconducting quantum circuit. The direction of isolation in an MPIJIS can be reversed in situ by negating the phase difference between the two pump tones driving the device.
A superconducting nondegenerate three-wave-mixing device can be used as part of the MPIJIS by operating the mixing device in a frequency conversion (no photon gain) mode. The nondegenerate three-wave mixer can be a Josephson parametric converter (JPC).
A superconducting nondegenerate three-wave mixer has 3 ports, which are Signal port (S) through which a microwave signal of frequency fS can be input, Idler port (I) through which an idler microwave signal of frequency fI can be input, and pump port (P) through which a microwave signal of frequency fP and phase φp can be input. In one configuration (without loss of generality), fI is a high frequency, fP is a low frequency, and fS is a medium frequency, when fP, fS, and fI are compared relative to each other (i.e., fI>fS>fP). The superconducting nondegenerate three-wave mixer is characterized as nondegenerate because it has two modes—namely S and I, which are both spatially and spectrally different.
From Idler to Signal port, the Idler microwave signal enters the Idler port at frequency f2, is down converted, and exits the Signal port at frequency f1. From Signal to Idler port, the Signal microwave signal enters the Signal port at frequency f1, is up converted, and exits the Idler port at frequency f2. The pump microwave signal provides the energy for frequency up conversion and frequency down conversion. The pump frequency is fP, where fP=fI−fS=f2−f1.
On resonance, the nondegenerate three-wave mixer (e.g., JPC) satisfies the following scattering matrix when operated in noiseless frequency conversion:
      [    S    ]    =      (                            r                          t                                                  t            ′                                    r                      )  
A nondegenerate three-wave mixer operates at a 50:50 Beam splitter working point where:
                            r                    2        =          1      2        ,                            t                    2        =                            1          2                ⁢                                  ⁢                  and          ⁢                                          [          S          ]                    =              (                                                            1                                  2                                                                                                      ie                                                            -                      i                                        ⁢                                                                                  ⁢                                          φ                      P                                                                                        2                                                                                                                          ie                                      i                    ⁢                                                                                  ⁢                                          φ                      P                                                                                        2                                                                                    1                                  2                                                                    )            
This mode is dependent upon the pump amplitude which has to be set appropriately for this mode to become operational. The phase of the pump φp (which are later denoted as φ1 and φ2 for two pump signals into two different nondegenerate three-wave mixers) will be utilized in accordance with the embodiments described herein. Phase φp is added to the signal propagating from port S to port I, and phase φp is subtracted from signal propagating from port I to port S.
Two suitable manifestations of the nondegenerate three-wave mixer, each operating in 50:50 beam splitting mode are used as one component in an MPIJIS according to the illustrative embodiments. JPC is one such non-limiting manifestation.
In quantum circuits, microwave signals can include more than one frequency. Generally, the microwave signals span a band of frequencies. An MPIJIS generally operates with a comparatively narrow band of frequencies around a central frequency for which the MPIJIS is tuned. The illustrative embodiments recognize that a new isolator design is needed that is capable of isolating all or some microwave signals having different, even if a frequency of a signal lies outside the operational frequency band of a single MPIJIS.